CSK IS CONSTITUTIVELY ASSOCIATED WITH A 60-KDA TYROSINE-PHOSPHORYLATED PROTEIN IN HUMAN T-CELLS

The protein-tyrosine kinase Csk is one of the main down-regulators of the Src family of kinases, Csk may be involved in the down-regulation of T cell receptor (TCR) signaling by C-terminal tyrosine phosphorylat ion of Lck and Fyn; however, it is not known how Csk activity is regul ated or how it targets these Src family members, We used Jurkat T cell s and normal human T cells to examine proteins that bind to the SH2 do main of Csk. In both Jurkat and normal T cells, the Src homology 2 (SH 2) domain of Csk bound constitutively to a tyrosine-phosphorylated pro tein of 60 kDa (p60), The 60-kDa protein was detected in Csk immunopre cipitates from both unstimulated and CD3-stimulated cells, In addition to p60, a protein of 190 kDa coprecipitated with Csk, and both protei ns were phosphorylated on tyrosine residues by the immunocomplex. Smal l amounts of GTPase-activating protein (GAP) were detected in anti-Csk immunoprecipitates, suggesting that p60 may be a GAP-associated prote in, Our data demonstrate that the SH2 domain of Csk specifically assoc iates with at least two tyrosine-phosphorylated proteins in normal hum an T cells, that this association is independent of TCR/CD3 activation , and that Csk may be a part of a multiprotein complex containing GAP.